Neither. Chromosomes cannot replicate or duplicate themselves.
In the S phase of interphase, the nuclear DNA is replicated in a process called DNA replication. A whole suite of proteins are involved, such as DNA helicase, single-stranded DNA binding proteins, DNA polymerase, primase, DNA ligase, topoisomerases, and so on. A chromosome cannot do this by itself.
The end result is that after the S phase of interphase, each 'chromosome' (chromatin fiber) has been duplicated so there are two double helices each. At the beginning of prophase the chromatin condenses into visible structures called chromosomes, and at that point it can be seen that each chromosome consists of 2 sister chromatids, joined together at their primary constriction, called a centromere.
PS: When being loose with terminology, people often use the terms "replicate" and "duplicate" interchangeably when speaking of what happens to 'chromosomes' just before cell division.
The proper term is called DNA replication, this is the interphase before cell division.
Defore condensing into chromosomes the cell goes through the interphase. The term for this is DNA replication where the DNA produces copies of itself. The DNA then condenses into 46 duplicated chromosomes, which then prepare themselves for cell division.
they replicate forming two chromatids joined by a centromere and condensed that they can be seen under a light microscope
Just prior to cell division, the chromosomes begin to replicate and condense.